Engine systems may utilize recirculation of exhaust gas from an engine exhaust system to an engine intake system, a process referred to as exhaust gas recirculation (EGR), to reduce regulated emissions. An EGR valve may be controlled to achieve a desired intake air dilution for the given engine operating conditions. Traditionally, the amount of low pressure EGR (LP-EGR) and/or high pressure EGR (HP-EGR) routed through the EGR system is measured and adjusted based on engine speed, engine temperature, and load during engine operation to maintain desirable combustion stability of the engine while providing emissions and fuel economy benefits. EGR effectively cools combustion chamber temperatures thereby reducing NOx formation. Also, EGR reduces pumping work of an engine resulting in increased fuel economy.
However, the inventors herein have recognized that the full fuel economy potential of EGR in an engine may be limited due to NVH issues associated with high EGR levels. In particular, engine control systems may operate the engine with sub-optimal levels of EGR to improve vehicle drivability and reduce operator dissatisfaction. One example approach for EGR usage is shown by Taplin et al. in U.S. Pat. No. 3,872,846. Therein, the engine control system controls EGR delivery in a closed-loop responsive to engine roughness, as determined by a tachometer coupled to a crankshaft driven member of the engine. In particular, EGR flow to an engine is reduced in response to detection of engine roughness in order to restrict further engine roughness, thereby maintaining driving comfort. However, by limiting the EGR levels to improve drivability and passenger comfort, the engine's ability to leverage EGR for attaining higher fuel economy is compromised.
The inventors herein have recognized that there may be conditions where drivability is reduced due to causes external to the vehicle, such as due to rough road conditions. During such conditions, the EGR level may be opportunistically raised since the operator may be unable to distinguish any NVH issues associated with the increased EGR levels from those associated with the rough road condition. In one example, the fuel economy potential of EGR usage can be improved using an example method for an engine comprising, in response to an indication of road roughness, selectively increasing an EGR flow rate to the engine. Road roughness conditions may be monitored using input from a plurality of sensors. These may include, as non-limiting examples, crankshaft acceleration sensors, wheel speed sensors, dynamic suspension system sensors including yaw-rate sensors, steering wheel sensors, etc. During smoother road conditions, EGR flow may be lowered from a target level in order to reduce potential NVH issues associated with higher EGR levels. In comparison, during rougher road conditions, EGR flow may be opportunistically raised to or above the target level. Likewise, during rough road conditions, one or more other engine operating parameters that improve fuel economy at the cost of combustion stability or NVH can be adjusted, such as by applying an increased purge frequency, advancing gear shift schedules, using less spark retard during gear shift schedules, adjusting torque convertor slip/lock schedules, adjusting knock and pre-ignition thresholds, and adjusting exhaust cam phasing in a variable cam timing (VCT) engine.
In this way, by increasing EGR delivery during conditions of elevated road roughness, higher engine fuel economy may be achieved without an additional increase in NVH for the passengers. By enabling NVH associated with elevated EGR levels (or elevated purge levels, for example) to be masked by NVH associated with rough road conditions, a higher EGR usage may be enabled, improving engine performance. As such, this may be particularly advantageous in global markets where road conditions are generally poor. The technical effect of adapting engine operating parameters responsive to rough road conditions to improve fuel economy is that higher fuel economy and improved emissions benefits may be achieved without any perceptible increase in NVH.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.